Partial Oxidation of Methane to Syngas Over γ‑Al₂O₃‑Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity

A series of supported Rh/γ-Al₂O₃ catalysts with an overall metal loading of 0.005 wt % was synthesized by impregnation of γ-Al₂O₃ with a toluene solution containing colloidally prepared well-defined (1.1, 2.5, 2.9, 3.7, and 5.5 nm) Rh nanoparticles (NP). The size of NP was not found to change after their deposition on γ-Al₂O₃ and even after performing partial oxidation of methane (POM) to synthesis gas at 1073 K for 160 h on stream. Apparent CO formation turnover rates and CO selectivity strongly decrease with an increase in this size. Contrarily, the overall scheme of POM is size-independent, i.e. CO and H₂ are mainly formed through reforming reactions of CH₄ with CO₂ and H₂O at least under conditions of complete oxygen conversion. The size effect on the activity and selectivity was related to the kinetics of interaction of CH₄, O₂, and CO₂ with Rh/γ-Al₂O₃ as concluded from our microkinetic analysis of corresponding transient experiments in the temporal analysis of products reactor. The rate constants of CH₄, O₂, and CO₂ activation decrease with an increase in the size of supported Rh NP thus influencing both primary (methane combustion) and secondary (reforming of methane) pathways within the course of POM

Investigation of the Enhancing Effect of Solid Cocatalysts on Propene Formation in Ethene/<i>trans</i>-2-Butene Metathesis over MoO_<i>x</i>/SiO₂–Al₂O₃

The metathesis of ethene and 2-butenes to propene over WO_<i>x</i>/SiO₂ is an important industrial process and has been intensely studied over alternative catalysts with supported MoO_<i>x</i> species. Such studies have, however, not analyzed the effect of cocatalysts on propene production. Here, we utilized CaO, Al₂O₃, or SiO₂–Al₂O₃ located upstream to MoO_<i>x</i>/SiO₂–Al₂O₃ (0–100 wt % SiO₂) to probe their influence on the rate of propene formation and on stream stability. All three prebed materials did not produce propene but significantly enhanced the metathesis activity of supported MoO_<i>x</i> species, with CaO having resulted in the highest increase. The strength of the effect was established to depend on the kind of MoO_<i>x</i> species and support material. From a mechanistic viewpoint, prebeds generate a gas-phase promoter from <i>trans</i>-2-butene but not from ethene, which further reacts with MoO_<i>x</i>-containing catalysts and increases their activity. The promoter is responsible for the stabilization and/or formation of catalytically active Mo–carbene species but not for the reduction of MoO^VI to MoO^IV. The obtained results provide new insights for the design of metathesis catalysts and open the possibility for tuning their activity and on-stream stability through the operation of prebed materials and metathesis catalysts in individual reactors at different temperatures

Steady-State and Transient Kinetic Studies of the Acetoxylation of Toluene over Pd–Sb/TiO₂

A combination of steady-state catalytic tests, transient studies with isotopic tracers, and kinetic modeling was used to derive detailed insights into the individual reaction pathways in the course of toluene acetoxylation over a Pd–Sb/TiO₂ catalyst. This reaction can be considered as an environmentally friendly route for the production of benzyl alcohol. Benzyl acetate and benzaldehyde are the only products formed from toluene, while acetic acid gives CO₂ in addition to benzyl acetate. The Arrhenius plots revealed apparent activation energies for formation of benzyl acetate and benzaldehyde of 24.9 and 27.5 kJ mol^–1, respectively, thus, indicating that these products originate from the same surface intermediate, i.e. benzyl cation. The corresponding value for CO₂ formation was 152.9 kJ mol^–1. Transient isotopic studies and their kinetic evaluation demonstrated the participation of lattice oxygen and adsorbed oxygen species in activation of acetic acid, with the latter species favoring oxidation of the acid to CO₂

Effect of Support and Promoter on Activity and Selectivity of Gold Nanoparticles in Propanol Synthesis from CO₂, C₂H₄, and H₂

Direct propanol synthesis from CO₂, H₂, and C₂H₄ was investigated over TiO₂- and SiO₂-based catalysts doped with K and possessing Au nanoparticles (NPs). The catalysts were characterized by scanning transmission electron microscopy and temperature-programmed reduction of adsorbed CO₂. Mechanistic aspects of CO₂ and C₂H₄ interaction with the catalysts were elucidated by means of temporal analysis of products with microsecond time resolution. CO₂, which is activated on the support, is reduced to CO by hydrogen surface species formed from gas-phase H₂ on Au NPs. C₂H₄ adsorption also occurs on these sites. In comparison with TiO₂-based catalysts, the promoter in the K–Au/SiO₂ catalysts was found to increase CO₂ conversion and propanol production, whereas Au-related turnover frequency of C₂H₄ hydrogenation to C₂H₆ decreased with rising K loading. The latter reason was linked to the effect of the support on the ability of Au NPs for activation of C₂H₄ and H₂. The positive effect of K on CO₂ conversion was explained by partial dissolution of potassium in silica with formation of surface potassium silicate layer thus inhibiting formation of potassium carbonate, which binds CO₂ stronger and therefore hinders its reduction to CO

The Role of Adsorbed and Lattice Oxygen Species in Product Formation in the Oxidative Coupling of Methane over M₂WO₄/SiO₂ (M = Na, K, Rb, Cs)

MnOx–Na2WO4/SiO2 is one of the best-performing catalysts in the oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H6 and C2H4). The current mechanistic concepts related to the selectivity to the desired products are based on the involvement of crystalline Mn-containing phases, the molten Na2WO4 phase, surface Na–WOx species, and the associated lattice oxygen. Using in situ X-ray diffraction, operando UV–vis spectroscopy, spatially resolved kinetic analysis of product formation in steady-state OCM tests, and temporal analysis of products with isotopic tracers, we show that these phases/species are not categorically required to ensure high selectivity to the desired products. M2WO4/SiO2 (M = Na, K, Rb, Cs) materials were established to perform similarly to MnOx–Na2WO4/SiO2 in terms of selectivity–conversion relationships. The unique role of the molten Na2WO4 phase could not be confirmed in this regard. Our alternative concept is that the activity of M2WO4/SiO2 and product selectivity are determined by the interplay between the lattice oxygen of M2WO4 and adsorbed oxygen species formed from gas-phase O2. This lattice oxygen cannot convert CH4 to C2H6 but oxidizes CH4 exclusively to CO and CO2. Adsorbed monoatomic oxygen species reveal significantly higher reactivity toward overall CH4 conversion and efficiently generate CH3 radicals from CH4. These reactive intermediates couple to C2H6 in the gas phase and are oxidized, to a lesser extent, by the lattice oxygen of M2WO4 to CO and CO2. Adsorbed diatomic oxygen is involved in the direct CH4 oxidation to CO2. The electronegativity of alkali metal in M2WO4 was established to affect the catalyst ability to generate adsorbed oxygen species from O2. This knowledge opens the possibility to influence product selectivity by controlling the coverage by adsorbed and lattice oxygen via reaction conditions or catalyst design